The purpose of this study was to 1) determine the acute and chronic toxicity of concurrent intravenous administration of 5-Fluorouracil (5-FU) and 5-IODO-2'-Dexoyuridine (IDUR); 2) to determine the maximum tolerated dose (MDT) of intravenous IUDR when administered during a continuous infusion of 5-FU; and 3) determine the pharmacokinetics of 5-FU and IUDR when these agents are given concurrently.